Once the Senate ratifies a treaty, that treaty is coequal with federal laws. The treaty is therefore to be considered the same way every federal law is treated, from speeding tickets on federal property to antitrust laws.
If you will recall, federal laws are subject to the US constitution. Accordingly, a treaty cannot give the federal government more power than it is otherwise allowed by the constitution, but a treaty may grant powers that are within the boundaries of what is constitutionally allowed.
International law is the term used to describe the framework of treaties to which countries belong. Each of the countries ratifies the treaty in a manner analogous to manner in which the US does, and subsequently, each member country is subject to international law to the extent permitted by their respective constitutions.
Because treaties are law in the US, it is quite possible for an act of the US government to be illegal in the traditional sense. If the treaty was duly adopted and its terms were abrogated, then those abrogating acts are ipso facto illegal. The issue is then whether there is a mechanism for enforcing the treaty against the offending party.
Congress cannot change treaties. They can repudiate them, but they cannot alter them to suit their tastes. The present administration would have us believe that they were acting in accordance with the UN treaty in invading Iraq. That is a total crock. The US can tell the UN to fuck itself, but without an explicit resolution giving the US the authority to invade a sovereign country, the US is in violation of the treaty and the invasion is therefore technically illegal. But again, since the UN could do nothing to stop the US in this case, the question is kind of moot isn’t it?
While the UN is/was powerless to stop the US actions, the US cannot now turn around and claim that their actions were perfectly legal. They were not.
CJ